Externally splined fastener

ABSTRACT

A fastener ( 2 ) for securing a workpiece comprising a plurality of workpiece members, said fastener comprising a shank ( 4 ) and a radially enlarged head ( 6 ) wherein the fastener further comprises axial voids ( 40 ) and splines ( 18 ), such that when the fastener is installed into a workpiece by a mandrel, breakstem or pin, the shank of the fastener is expanded such that the crests of the splines are caused to mechanically engage with the internal walls of the fastener apertures.

CROSS-REFERENCE TO RELATED APPLICATIONS

The application is a national phase filing of International Application No. PCT/GB2011/051107, filed on Jun. 14, 2011, which claims priority to Great Britain Application No. 1012298.4, filed on Jul. 22, 2010, each of which is incorporated herein by reference.

BACKGROUND AND SUMMARY

This invention relates to a speed fastener for securing apertured workpiece members together.

Speed fastening™ is a well known method of securing workpiece members together, whereby a fastener having a hollow core is placed in aligned apertures in the workpiece members, and a headed mandrel is pulled through the core of the fastener to cause radial expansion of the fastener shank, and additionally a small degree of foreshortening of the fastener. The radial expansion of the fastener shank ideally achieves mechanical engagement between the fastener shank and the walls of the apertures in the workpiece members. The axial foreshortening effect can help pull the rearmost sheet up towards the fastener head to close gaps between the workpiece members.

Prior art fasteners, such as those available under the trade marks Briv (UK patent number GB1323873A) and Chobert, have cylindrical shank portions which in a central region have a uniform wall thickness, i.e. a constant cross-sectional area. The tail end of the fastener shank has an increased wall thickness; in the case of the Briv fastener this is an enlarged outer diameter and in the case of the Chobert fastener, a reduced bore diameter of tapered form.

These prior art fasteners are installed by pulling a mandrel of a given diameter through the fastener bore, thereby causing a radially enlarged head of the mandrel to expand the central region of the shank into the apertures in the workpiece members. Accordingly, the degree of radial expansion is governed by the diameter of the mandrel head for a nominal fastener diameter.

Prior art fasteners have a low degree of tolerance in workpiece aperture diameter, i.e. only a limited range of workpiece aperture diameters can be accommodated with a nominal fastener size and a single head diameter mandrel. Accordingly, there is a narrow range between the two acceptable extremes of workpiece aperture diameter when using a nominal fastener and a specific head diameter mandrel.

In an optimised installation of a prior art fastener, i.e. wherein the workpiece aperture diameters are within the acceptable range for a given fastener diameter and mandrel head diameter, the expansion of the fastener shank by the mandrel head causes mechanical engagement between the fastener shank and the walls of the workpiece apertures, without ‘overpacking’ (as explained below). The constraint provided by the wall of the aperture of the rear workpiece member (i.e. the workpiece member which is furthest away from the fastener head) generates radial forces and hence frictional resistance to axial movement of the rear workpiece member relative to the fastener rivet shank.

However, if a workpiece aperture diameter is smaller than the acceptable range for a prior art fastener, an unduly high placing force will be required to install the fastener because it will be overpacked in the aperture. This can cause wear of the mandrel or cause other undesirable effects such as debris generation from the fastener bore, or fastener head malformation, such as by extrusion of fastener bore material up into the head region.

It is common for the aperture diameter of the top workpiece member (i.e. the workpiece member which is closest to the fastener head) to be configured such that it is slightly larger than the aperture in the rear workpiece member, in order to accommodate aperture pitch errors. If the aperture diameter of the top workpiece member is above the acceptable range for a prior art fastener, the expansion of the shank of the fastener does not result in any mechanical engagement between the fastener shank and the wall of the oversized workpiece aperture, resulting in low resistance of the joint to movement under shear loads.

To compensate for oversize workpiece apertures, an oversized mandrel, i.e. a mandrel having an oversized head, can be used to install a fastener. However, there is a risk that an oversized mandrel can incorrectly be used for a undersized workpiece aperture, thereby leading to the problems caused by a high placing force as discussed above.

The problem of a workpiece aperture diameter being above the acceptable range for a specific fastener diameter and mandrel head diameter is illustrated in FIG. 1, which illustrates a prior art fastener 2 installed into a workpiece 30 at the lowest end of a grip range. The workpiece comprises a top workpiece member 32 having an oversized aperture 36, and a rear workpiece member 34 having an acceptable aperture 38. Due to the top workpiece member 32 having an oversized aperture 36, the installation has not resulted in any mechanical engagement between the fastener shank 4 and the wall of the aperture 36.

A correctly installed prior art fastener exhibits a enlarged tail end formation due to the greater wall thickness in that region. In a maximum-grip application, this enlarged tail end formation is adjacent to the outer surface of the rear workpiece member and so serves to provide support and resistance to movement of the joint if subject to tensile loads. However in a mid-grip or a minimum-grip condition (as shown in FIG. 1) the enlarged tail formation 44 is remote from the rear workpiece member 34 and it is therefore unable to help resist initial separation of the workpiece members 32, 34 under tensile loads; the resistance must result almost entirely from the frictional forces resulting from the radial pressure exerted by the expanded central region of the fastener shank 4 having a uniform wall thickness. Consequently the resistance to separation is lower in mid or minimum-grip conditions.

Other speed fasteners which are currently available, such as those available under the trade marks Avtronic™ and Rivscrew™, feature a non-uniform wall thickness in the centre of the fastener shank. These fasteners comprise voids on the exterior of the fastener shank in the form of annular grooves and a helical screw thread form respectively. These fasteners are intended to expand and embed into a workpiece wherein the workpiece members are comprised of a material which is relatively soft in comparison to the material of the fastener. The annular or helical voids of these fasteners lead to variable expansion into the rear workpiece member, with high or low degrees of mechanical engagement between the fastener shank and the wall of the workpiece apertures, and an expanded tail form of inconsistent size or distribution around the hole periphery.

The present invention is aimed at providing a fastener which will ensure complete and consistent mechanical engagement between the fastener shank and the wall of both the workpiece apertures, i.e. achieve complete mechanical engagement for a fastening which is more robust than is achievable with prior art fasteners. The present invention is also aimed at providing a fastener which allows the specification of a single fastener length and one mandrel size to install into a wider grip range and which can tolerate a far greater variation in workpiece aperture diameter than prior art fasteners, whilst avoiding the problems of high placing force, mandrel wear, debris generation and fastener head malformation with a single mandrel head size or diameter.

Furthermore the present invention is aimed at providing a fastener which is suitable for use in securing workpiece members which may be of harder material than the fastener, and wherein the grip range of the fastener is dependent only upon the length of the fastener i.e. a longer fastener has a greater grip range, allowing a fastener having a nominal length to be used in a wider range of workpiece thicknesses.

Accordingly the present invention comprises, in a first aspect, a fastener, for securing a workpiece comprising a plurality of workpiece members, said fastener comprising a shank with a head end and a tail end remote from the head end, and a radially enlarged head at the head end of the shank, and a bore extending throughout the fastener, wherein an external wall of the shank comprises a voided section comprising a plurality of voids separated by a plurality of splines each of which culminates in a crest, the voids and splines being elongated in an axial direction with respect to a longitudinal axis of the fastener bore; wherein the voids are of an approximate trapezoidal cross-section, and wherein the voids have a cross-sectional area which is constant along the length of the voided section of the fastener shank.

The present invention comprises, in a further aspect, a method of installation of a fastener, thereby to secure a workpiece comprising a first apertured workpiece member and a second apertured workpiece member, the method comprising the steps of; a) placing a mandrel having a radially enlarged head through the fastener bore such that the radially enlarged head of the mandrel is adjacent the tail end of the fastener shank; b) inserting the fastener and mandrel into the apertures in the first and second workpiece members such that the fastener head contacts the first workpiece member; c) supporting the fastener at the head end whilst drawing the mandrel entirely through the fastener bore, thereby expanding the fastener shank into the apertures of the workpiece members, and causing the crests of the splines to be deformed against the apertures, and causing the tail end of the fastener shank to radially enlarge adjacent to the second workpiece member.

The installation of the fastener may be undertaken by a tool comprising a conical concave end face which bears on an upper surface of the fastener head. This causes the fastener head to be pushed towards the top workpiece member and the head to flatten slightly against the top workpiece member, and thereby ensuring clamping of the fastening. The flattening of the head periphery against the top workpiece member acts such that the outer diameter of the underhead recess decreases, and the bearing area of the fastener head against the workpiece is increased. Furthermore the bearing area of the tool end face on the upper surface of the fastener head is greater than with prior art speed fasteners such that undesirable indentations or damage to the surface of the fastener head are minimised during installation of the fastener.

The present invention comprises, in further aspects, a method of installation of a fastener, thereby to secure a workpiece comprising a first apertured workpiece member and a second apertured workpiece member, the method comprising the steps of; a) inserting a stem comprising a radially enlarged head, a breakneck point, and a plugging portion, into the fastener through the fastener bore such that the radially enlarged head of the stem is adjacent the tail end of the fastener shank; b) inserting the fastener and the stem into the apertures in the first and second workpiece members such that the fastener head contacts the first workpiece member; supporting the fastener at the head end whilst pulling the stem with respect to the fastener head, thereby causing the plugging portion to radially expand the fastener shank into the apertures of the workpiece members and deform the crests of the splines against the apertures, and also thereby causing the radially enlarged head of the stem to contract, and then compress, the tail end of the fastener shank, thereby causing the tail end of the fastener shank to radially enlarge adjacent to the second workpiece member; wherein the stem is pulled with respect to the fastener head through the fastener bore until the stem fractures at the breakneck point or, thereby to secure a workpiece comprising a first apertured workpiece member and a second apertured workpiece member, the method comprising the steps of; a) inserting the fastener into the apertures in the first and second workpiece members such that the fastener head contacts the first workpiece member; b) inserting a breakstem comprising a plugging portion with a hollow core, an elongate shank, and a breakneck point between the plugging portion and the elongate shank, into the bore of the fastener, such that the plugging portion of the breakstem contacts the tail end of the fastener shank; c) pulling the elongate shank of the breakstem pulled relative to the fastener, thereby causing the breakstem plugging portion to enter the fastener shank, causing the fastener shank to expand into the workpiece apertures, and causing the crests of the splines to be deformed against the apertures, and simultaneously causing the plugging portion to collapse inwardly; wherein the elongate shank is pulled relative to the fastener until the breakstem fails at the breakneck point.

A fastener installed by the method of installing a fastener, thereby to secure a workpiece comprising a first apertured workpiece member and a second apertured workpiece member, the method comprising the steps of; a) inserting the fastener into the apertures in the first and second workpiece members such that the fastener head contacts the first workpiece member; b) inserting a breakstem comprising a plugging portion with a hollow core, an elongate shank, and a breakneck point between the plugging portion and the elongate shank, into the bore of the fastener, such that the plugging portion of the breakstem contacts the tail end of the fastener shank; c) pulling the elongate shank of the breakstem pulled relative to the fastener, thereby causing the breakstem plugging portion to enter the fastener shank, causing the fastener shank to expand into the workpiece apertures, and causing the crests of the splines to be deformed against the apertures, and simultaneously causing the plugging portion to collapse inwardly; wherein the elongate shank is pulled relative to the fastener until the breakstem fails at the breakneck point may further comprise a parallel portion between the breakneck point and the plugging portion, which, during installation, forms a locking skirt thereby providing a mechanical lock of the installed stem in the fastener shank.

The present invention comprises, in a further aspect, a method of installation of a fastener, thereby to secure a workpiece comprising a first apertured workpiece member and a second apertured workpiece member, the method comprising the steps of; a) inserting the fastener into the apertures in the first and second workpiece members such that the fastener head contacts the first workpiece member; b) driving a solid pin, having a maximum diameter larger than that of the fastener bore, into the bore of the fastener via the head thereby to radially enlarge the fastener shank into mechanical engagement with the workpiece apertures, and causing the crests of the splines to be deformed against the apertures. This method may further comprise a subsequent step of bearing on the fastener head by a support sleeve, and removing the pin from the fastener bore.

The axial voids of the present invention allow a variable degree of mechanical engagement between the fastener shank and the wall of the workpiece apertures in different sizes of workpiece member aperture, such as an oversized aperture in the top workpiece member, thereby providing mechanical resistance to shear movement, without leading to ‘overpacking’, thereby avoiding potentially excessive placing loads, mandrel wear, debris generation and fastener head malformation.

Because the voids are positioned axially with respect to the fastener shank, they have a cross-sectional area which is constant along the length of the fastener shank. This ensures that the degree of radial expansion and the mandrel load are consistent, independent of grip thickness. The present invention can therefore be used in a greater grip range than that offered by equivalent-sized prior art fasteners. Furthermore the grip range which can be accommodated by a specific fastener is dependent only upon the length of the fastener, i.e. a longer fastener can accommodate a greater grip range.

Furthermore, the present invention provides that a single mandrel (i.e. a mandrel having a set head size) can be used to install a fastener into a workpieces having a variety of aperture sizes.

The present invention also provides greater expansion of the tail end of the fastener shank adjacent to the rear workpiece member, throughout the grip range, thereby providing increased resistance to separation of the workpiece members under tensile loads.

The present invention is also suitable for use in securing workpiece members which may be of harder material than the fastener.

The axial voids are preferably equidistant from one another.

The axial voids preferably extend from immediately below the fastener head.

The fastener head may include a countersink in its top surface, the countersink having an average diameter which is greater than the diameter of the bore of the fastener. The countersink could comprise a flat or a curved annular tapered wall, and a neck portion could be provided between the countersink and the fastener bore. The countersink and the neck portion both accommodate rivet body material which is drawn axially through and into the rivet head by the passage of the enlarged mandrel head through the rivet bore. Furthermore, the countersink controls the broach load on installation of the fastener, and avoids excess fastener material pulling out from the top of the installed fastener head.

The fastener may include a tapered point at the tail end of the fastener shank. An advantage of the tapered point feature is that it allows fasteners to be nested together “head-to-tail” in a stack within a paper pod, etc, when a countersink is provided in the fastener head. This ensures coaxial alignment of adjacent rivet bores which in turn makes it easier to load the fastener stack as one onto a mandrel shank. Furthermore the tapered point also assists in locating the fastener in the workpiece aperture, and reduces the height of the stack of fasteners prior to installation, thus permitting more rivets to fit a placing tool of a given length, and provides increased rigidity to the stack on the relatively flexible mandrel.

The fastener may also include a solid tail ring provided at the tail end of the fastener shank, between the voided section and the tapered point. The tail ring, despite its remote position from the rear workpiece member, nonetheless contributes to the ultimate tensile strength of the fastener via its reinforced wall section.

The fastener may also comprise an end section at the tail end of the fastener beyond the tapered point, the end section having a straight annular wall which is parallel to a longitudinal axis of the fastener. The end section is further facilitates the storing of multiple fasteners “head-to-tail” in a stack prior to installation.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described by way of example only and with reference to the figures in which;

FIG. 1 is a side view of a prior art fastener installed into a workpiece;

FIG. 2 is a side view of a fastener in accordance with the present invention prior to installation into a workpiece;

FIGS. 3 a and 3 b are an isometric views of the fastener of FIG. 2;

FIG. 4 is a cross-sectional view of the fastener of FIG. 2 along the line IV-IV;

FIG. 5 is a cross-sectional view of the fastener of FIG. 4 along the line V-V;

FIG. 6 is a cross-sectional view of the fastener of FIG. 2 along the line VI-VI;

FIG. 7 is a side view of a plurality of fasteners of FIG. 2 arranged in a stack;

FIG. 8 is cross-sectional view of the stack of fasteners of FIG. 7 along the line VIII-VIII;

FIG. 9 a is an axial cross-sectional view of the fastener of FIG. 2 installed into a workpiece wherein the aperture diameter of the top workpiece member is oversized;

FIG. 9 b is an axial cross-sectional view of the fastener of FIG. 2 installed into a workpiece having a minimum grip;

FIG. 10 a is a cross-sectional view of the installed fastener of FIG. 9 a along the line Xa-Xa;

FIG. 10 b is a cross-sectional view of the installed fastener of FIG. 9 b along the line Xb-Xb;

FIG. 11 is a cross-sectional view of the installed fastener of FIG. 9 a along the line XI-XI;

FIG. 12 is a cross-sectional view of the installed fastener of FIG. 9 a along the line XII-XII;

FIGS. 13 and 14 illustrate an alternative method of installation of the fastener of FIG. 2;

FIGS. 15 and 16 illustrate a further alternative method of installation of the fastener of FIG. 2;

FIG. 17 illustrates a further alternative method of installation of the fastener of FIG. 2;

FIG. 18 illustrates a alternative embodiment of fastener in accordance with the present invention;

FIG. 19 is a cross-sectional view of the fastener of FIG. 18 along the line XIX-XIX;

FIG. 20 is a side view of a further alternative embodiment of fastener in accordance with the present invention;

FIG. 21 is a cross-sectional view of the fastener of FIG. 20 along the line XXI-XXI;

FIG. 22 is an isometric view of the fastener of FIG. 20;

FIG. 23 is a side view of a plurality of fasteners of FIG. 20 arranged in a stack;

FIG. 24 is cross-sectional view of the stack of fasteners of FIG. 23 along the line XXIV-XXIV; and

FIG. 25 is cross-sectional view of a fastener, workpiece, tool nose (remaining tool is omitted for clarity) and mandrel in accordance with the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring to FIGS. 2 to 8, a fastener 2 according to the present invention comprises a shank 4, a radially enlarged head 6 with a domed upper surface 7 at a head end 8 of the shank 4, and a tapered point 10 at a tail end 12 of the shank 4. An annular underhead recess 42 is provided under the head 6. A bore 14, having a diameter B, is provided throughout the fastener 2, i.e. throughout the shank 4, head 6 and tapered point 10. A voided section 16 is provided on the external wall 20 of the shank 4. The voided section 16 comprises a plurality of substantial axial voids 40, which extend radially inwardly from the major diameter ‘D’ of the shank exterior, and which separate a plurality of substantial axial splines 18, which extend radially outwardly from the minor diameter ‘d’ of the shank exterior. The splines 18 are parallel and are spaced equidistant from one another around the external wall 20 of the shank 4, and extend axially, i.e. with respect to a longitudinal axis of the fastener bore 14, from under the head 6 towards the tail end 12 of the shank 4. A tail ring 22 is provided at the tail end 12 of the shank 4, between the voided section 16 and the tapered point 10. The tail ring 22 comprises a solid wall, i.e. the voided section does not extend into the tail ring 22. A tapered transition section is provided between the voided section 16 and the tail ring 22, whereby at the end of each void 40 further from the head 6, the shank gradually increases in diameter to the major diameter ‘D’ of the shank exterior.

The splines 18 are of an approximate trapezoidal cross-section, and culminate in flattened crests 24, wherein a maximum shank diameter ‘D’ is defined between the centres of opposing crests 24, and a minimum shank diameter ‘d’ is defined between the centres of opposing voids 40. The depth of the voids 40 (i.e. the height H of the spines 18 as shown in FIG. 4) is greater than would be present, for example, in knurling. The ratio of spline height H to maximum shank diameter D could be, for example, approximately between 1:6 and 1:14.

A countersink 26 comprising a flat annular tapered wall is provided in the top surface 5 of the head 6. The countersink 26 comprises a flat, annular tapered wall, such that the countersink has a maximum diameter X furthest from the fastener bore, and a minimum diameter x closest to the fastener bore. The average diameter of the countersink is therefore greater than the diameter B of the fastener bore. A tapered neck portion 28 is provided between the countersink 26 and the fastener bore 14, wherein the maximum diameter Y of the neck portion 28 is equal to the minimum diameter x of the countersink, and the minimum diameter y of the neck portion 28 is equal to the diameter B of the fastener bore.

Multiple fasteners are provided “head-to-tail” in a stack 50 as illustrated in FIGS. 7 and 8, ready for loading onto the mandrel or for sequential installation once on a mandrel within the placing tool. The maximum diameter X of the countersink, and the angle of taper of the countersink 26, are such that the tapered point 10 of an adjacent fastener can be accommodated at least partially in the countersink 26, allowing the fasteners to be stacked.

Installation of the fastener 2 to secure a workpiece 30 comprising a first, top workpiece member 32 and a second, rear workpiece member 34 (i.e. the workpiece member which is furthest away from the fastener head) is effected by inserting a mandrel 104 (FIG. 25) having a radially enlarged head 106 of maximum diameter M (where M is greater than the diameter B of the fastener bore 14), through the bore 14 of the fastener 2, such that the radially enlarged head 106 of the mandrel is at the tail end 12 of the fastener shank 4. The fastener 2 and mandrel 100 are inserted into apertures 36, 38 provided in the workpiece members 32, 34. A tool having a tool nose 100 (the nose of the tool only is shown in FIG. 25) having a conical concave end face 102 is used to bear on the domed upper surface 7 of the fastener head 6. The tool nose 100 is used to pull the mandrel 104 through the fastener bore 14, thereby causing the fastener shank 4 to expand and cause mechanical engagement between the crests 24 of the splines 18 and the walls of the workpiece apertures 36, 38 as the crests 24 are crushed against the walls of the apertures 36, 38. The countersink 26 and the neck portion 28 both accommodate fastener body material which is drawn axially through and into the fastener head 6 by the passage of the enlarged mandrel head 106 through the fastener bore 14. Furthermore, the countersink controls the broach load on installation of the fastener 2, and avoids excess fastener material pulling out from the top of the installed fastener head 6.

FIG. 9 a illustrates the fastener 2 fully installed into a workpiece 30 having a maximum grip, and FIG. 9 b illustrated the fastener 2 fully installed into a workpiece 30 having a minimum grip.

The force applied by the conical concave end face 102 of the tool nose 100 to the domed upper surface 7 of the fastener head 6 causes the head 6 to be pushed towards the top workpiece member 32 and the head to flatten slightly against the top workpiece member 32, and thereby ensuring clamping of the fastening. The flattening of the head periphery 22 against the top workpiece member 32 acts such that the outer diameter of the underhead recess 42 decreases, and the bearing area of the fastener head 6 against the workpiece 30 is increased. Furthermore the bearing area of the tool end face 102 on the upper surface 7 of the fastener head 6 is greater than with prior art speed fasteners such that undesirable indentations or damage to the surface of the fastener head 6 are minimised during installation of the fastener.

The extent to which the crests 24 are crushed against the a wall of a workpiece apertures 36, 38 during installation is determined by the diameter of the aperture. An oversize workpiece aperture will result in the crests 24 being only slightly crushed against the wall of the workpiece aperture 36, 38. This is illustrated in FIG. 11, which illustrates marginal crushing of the crests 24 against the wall of the aperture 36 of the top workpiece member 32. As illustrated in FIG. 12, an aperture having a nominal diameter (i.e. which is smaller than the oversized aperture of FIG. 11), will result in a greater degree of crushing of the crests 24 against the wall of the workpiece aperture 38 than occurs with the oversized aperture of FIG. 11.

The displaced material of the crushed crests 24 is accommodated by the axial voids 40. For a minimum diameter workpiece aperture, the axial voids 40 will be completely filled by displaced crest material.

The tail ring 22 helps support the ends of the splines 18 and prevents them from folding “inwards” under tensile joint loads. After the initial resistance of the crest 24 of the spline 18 is overcome in very high tensile loading, the crest 24 are “peeled off” as the rear sheet slides up to the expanded tail ring. At the tail ring 22, the resistance to further movement is higher as this portion is a solid ring, therefore ensuring the ultimate tensile strength of the installed fastener.

It is possible that fastener 2 and/or that the aperture 38 of the rear workpiece member 34 are not concentric with the aperture 36 of the top workpiece member 32. In this situation, the crests 24 of the splines 18 will be crushed by a variable amount against the wall of the aperture 36, thereby still ensuring mechanical engagement of all crests 24 against the aperture wall, despite the eccentricity of the fastener 2 and/or the rear workpiece member aperture 38 relative to and top workpiece member aperture 36.

FIGS. 9 a to 10 b also illustrate an enlarged tail formation 44 which results from the installation of the fastener 2. Beyond the rear workpiece member 34, the splines 18 are free to expand fully (see FIGS. 10 a and 10 b), thus creating a ‘footprint’ or enlarged tail formation 44 which is adjacent to the rear workpiece member 34 and which is in contact with the rear workpiece member 34 (see FIG. 10 a). A plurality of trapezoidal footprint contact areas 46 are formed. Because the enlarged tail formation 44 is formed adjacent to and in contact with the rear workpiece member 34, it therefore provides a greatly increased mechanical resistance to initial axial movement due to tensile loading on the fastening than is provided by prior art fasteners. The enlarged tail formation 44 is formed adjacent to the rear workpiece member 34 across a given grip range, as can be seen in FIGS. 9 a and 9 b.

In the above embodiment, the axial voids are provided by a plurality of splines of trapezoidal cross-section. However, in an alternative embodiment, the axial voids may be of a different cross-section.

In the embodiment described above, the fastener is installed by pulling a radially enlarged mandrel head completely through the fastener bore 14. However, alternative methods of expanding the fastener shank 4 thereby to install the fastener 2 are illustrated in FIGS. 13 to 17. The method illustrated in FIGS. 13 and 14 uses a solid breakstem 70, which comprises a breakneck point 72, a radially enlarged head 74, and plugging portion 76 between the breakneck point 72 and the head 74, wherein the plugging portion 76 is of an equivalent diameter to the maximum diameter M of the mandrel head 106 of the used in the first embodiment. The breakstem 70 and fastener 2 are inserted into the apertures 36, 38 in the workpiece members 32, 34. The breakstem 70 is subsequently pulled through the fastener 2 causing the plugging portion 76 to radially expand the shank 4 of the fastener, until the radially enlarged head 74 of the breakstem 70 contacts with the tail end 12 of the fastener shank 4, which causes the load being applied to the stem 70 to increase sufficiently to cause the stem 70 to fail at the breakneck point 72 FIG. 14 illustrates the fastener 2 fully installed into the workpiece 30. The plugging portion 76 remains within the shank 4 of the fastener 2 and contributes additionally to the ultimate shear strength of the installed fastener. Supplementary “barbs”, knurls or similar (not shown) could be provided on the plugging portion outer surface to provided enhanced resistance against recoil of the stem plugging portion 76 on failure of the breakneck point 72 and against removal of the plugging portion 76 from the fastener 2 after installation.

The alternative method illustrated in FIGS. 15 and 16 uses a breakstem 80 such as that available under the trade mark Monobolt (UK patent number GB1538872). The breakstem comprises a plugging portion 82 with a hollow core 84, a parallel portion 85 adjacent the plugging portion 82, an elongate shank 88, and a breakneck point 86 between the parallel portion 85 and the elongate shank 88. Installation of the fastener into a workpiece 30 involves inserting the fastener into the apertures 36, 38 in the workpiece members 32, 34, the breakstem is inserted into the bore 14 of the shank 4, such that the plugging portion of the breakstem contacts the tail end 12 of the fastener shank 4. The elongate shank 88 is then pulled relative to the fastener 2 thereby causing the breakstem plugging portion 82 to enter the fastener shank 4, simultaneously causing the fastener shank 4 to expand and the plugging portion 82 to collapse inwardly to a small degree due to the hollow core 84. The elongate shank 88 is pulled until the breakstem 80 fails at the breakneck point 86.

The slight inward collapse of the plugging portion 82 allows for a variation in workpiece aperture dimension, i.e. the plugging portion 82 is more compliant than the solid core of the breakstem 70 of FIGS. 13 and 14. This method of installation therefore provides an enhanced tolerance to differences in workpiece aperture diameter. The breakstem 80 could include a locking “skirt” feature (not shown) which is created by the parallel portion 85 on installation of the fastener, wherein material of the parallel portion 85 is displaced radially outwards into the countersink in the fastener head 6, i.e. into the area marked at 87 in FIG. 16. The locking skirt thereby provides a mechanical lock of the installed breakstem 80 in the fastener shank 4 to resist recoil of the plugging portion 82 on failure of the breakneck point 86 and against removal of the plugging portion 86 from the fastener shank 4 after installation.

The method illustrated in FIG. 17 is a “push broach” method, wherein a solid pin 90, having a diameter larger than that of the fastener bore 14, is driven into the fastener 2 via the head 6 to expand the fastener shank 4. This pin 90 could remain in the fastener shank 4 permanently or could be a removable part of the placing tooling, in which case a support sleeve (not shown) might be needed around the pin 90 to bear on the fastener head 6 to allow removal of the pin 90 from the fastener shank 4.

In the alternative fastener 2′ as illustrated in FIGS. 18 and 19, the countersink 26′ comprises a curved tapered annular wall, and the tapered neck portion 28 is not present. This embodiment radius reduces the likelihood of debris generation on installation of the fastener. The broach load caused when the mandrel head passes through the fastener head 6 is reduce, whilst maintaining sufficient tensile strength. During installation, some material of the fastener shank 4 is displaced backwards towards the tool, such that the counterbore is no longer visible in the placed fastener.

A further alternative fastener (not shown in the figures) could comprise a countersink having a curved tapered annular, and a neck portion.

In a further alternative fastener 2″, as illustrated in FIGS. 20 to 22, an end section 60 having a straight annular wall which is parallel to the longitudinal axis of the fastener bore 14, is provided beyond the tapered point 10 at the tail end of the fastener shank 4. In this embodiment, the countersink 26″ comprises a flat annular wall which is parallel to the longitudinal axis of the fastener bore 14, i.e. the wall of the countersink 26″ is not tapered. A tapered neck portion 28 is also provided between the countersink 26″ and the fastener bore 14. The diameter of the countersink 26″ is similar to the diameter of the end section 60 to allow the fasteners to be arranged “head-to-tail” in a stack 50′ of the fasteners, as illustrated in FIGS. 23 and 24. 

What is claimed:
 1. A fastener, for securing a workpiece comprising a plurality of workpiece members, said fastener comprising a shank with a head end and a tail end remote from the head end, and a radially enlarged head at the head end of the shank, and a bore extending throughout the fastener, wherein an external wall of the shank comprises a voided section comprising a plurality of voids separated by a plurality of splines each of which culminates in a crest, the voids and splines being elongated in an axial direction with respect to a longitudinal axis of the fastener bore; wherein the voids are of an approximate trapezoidal cross-section, and wherein the voids have a cross-sectional area which is constant along the length of the voided section of the fastener shank.
 2. A fastener as claimed in claim 1 wherein the axial voids are equidistant from one another.
 3. A fastener as claimed in claim 1 wherein the axial voids extend from immediately underneath the fastener head.
 4. A fastener as claimed in claim 1 wherein the fastener head comprises a countersink provided in a top surface of the head remote from the fastener shank, the countersink having an average diameter which is greater than the diameter of the bore of the fastener.
 5. A fastener as claimed in claim 4 wherein the countersink comprises a flat annular tapered wall.
 6. A fastener as claimed in claim 4 wherein the countersink comprises a curved tapered annular wall.
 7. A fastener as claimed in claim 4 wherein the countersink comprises a flat annular wall which is parallel to the longitudinal axis of the fastener bore of the bore.
 8. A fastener as claimed in claim 4 wherein a tapered neck portion is provided between the countersink and the fastener bore.
 9. A fastener as claimed in claim 1 further comprising a tapered point at the tail end of the fastener shank.
 10. A fastener as claimed in claim 9 further comprising an end section at the tail end of the fastener beyond the tapered point, the end section having a straight annular wall which is parallel to a longitudinal axis of the fastener.
 11. A fastener as claimed in claim 10, wherein the diameter of the end section is less than to the diameter of the countersink.
 12. A fastener as claimed in claim 1 further comprising a solid tail ring at the tail end of the fastener shank, between the voided section and the tapered point.
 13. A method of installing a fastener according to claim 1, thereby to secure a workpiece comprising a first apertured workpiece member and a second apertured workpiece member, the method comprising the steps of; a) placing a mandrel having a radially enlarged head through a fastener bore such that the radially enlarged head of the mandrel is adjacent a tail end of a fastener shank; b) inserting the fastener and mandrel into the apertures in the first and second workpiece members such that the fastener head contacts the first workpiece member; c) supporting the fastener at a head end whilst drawing the mandrel entirely through the fastener bore, thereby expanding the fastener shank into the apertures of the workpiece members, and causing the crests of the splines to be deformed against the apertures, and causing the tail end of the fastener shank to radially enlarge adjacent to the second workpiece member.
 14. A method as claimed in claim 13 wherein step c) is undertaken by a tool comprising a conical concave end face which bears on an upper surface of the fastener head.
 15. A method of installing a fastener according to claim 1, thereby to secure a workpiece comprising a first apertured workpiece member and a second apertured workpiece member, the method comprising the steps of; a) inserting a stem comprising a radially enlarged head, a breakneck point, and a plugging portion, into the fastener through the fastener bore such that the radially enlarged head of the stem is adjacent the tail end of the fastener shank; b) inserting the fastener and the stem into the apertures in the first and second workpiece members such that the fastener head contacts the first workpiece member; c) supporting the fastener at the head end whilst pulling the stem with respect to the fastener head, thereby causing the plugging portion to radially expand the fastener shank into the apertures of the workpiece members and deform the crests of the splines against the apertures, and also thereby causing the radially enlarged head of the stem to contract, and then compress, the tail end of the fastener shank, thereby causing the tail end of the fastener shank to radially enlarge adjacent to the second workpiece member; wherein the stem is pulled with respect to the fastener head through the fastener bore until the stem fractures at the breakneck point.
 16. A method of installing a fastener according to claim 1, thereby to secure a workpiece comprising a first apertured workpiece member and a second apertured workpiece member, the method comprising the steps of; a) inserting the fastener into the apertures in the first and second workpiece members such that the fastener head contacts the first workpiece member; b) inserting a breakstem comprising a plugging portion with a hollow core, an elongate shank, and a breakneck point between the plugging portion and the elongate shank, into the bore of the fastener, such that the plugging portion of the breakstem contacts the tail end of the fastener shank; c) pulling the elongate shank of the breakstem pulled relative to the fastener, thereby causing the breakstem plugging portion to enter the fastener shank, causing the fastener shank to expand into the workpiece apertures, and causing the crests of the splines to be deformed against the apertures, and simultaneously causing the plugging portion to collapse inwardly; wherein the elongate shank is pulled relative to the fastener until the breakstem fails at the breakneck point.
 17. A method of installing a fastener according to claim 15, wherein the stem further comprises a parallel portion between the breakneck point and the plugging portion, and wherein, during step c), a locking skirt is created by the parallel portion, thereby providing a mechanical lock of the installed stem in the fastener shank.
 18. A method of installing a fastener according to claim 1, thereby to secure a workpiece comprising a first apertured workpiece member and a second apertured workpiece member, the method comprising the steps of; a) inserting the fastener into the apertures in the first and second workpiece members such that the fastener head contacts the first workpiece member; b) driving a solid pin, having a maximum diameter larger than that of the fastener bore, into the bore of the fastener via the head thereby to radially enlarge the fastener shank into mechanical engagement with the workpiece apertures, and causing the crests of the splines to be deformed against the apertures.
 19. A method of installing a fastener according to claim 18 further comprising a subsequent step of bearing on the fastener head by a support sleeve, and removing the pin from the fastener bore.
 20. (canceled)
 21. Fastening apparatus, for securing a workpiece comprising a plurality of workpiece members, said apparatus comprising a fastener and a mandrel; said fastener comprising a shank with a head end and a tail end remote from the head end, and a radially enlarged head at the head end of the shank, and a bore extending throughout the fastener; wherein an external wall of the shank comprises a voided section comprising a plurality of voids separated by a plurality of splines each of which culminates in a crest, the voids and splines being elongated in an axial direction with respect to a longitudinal axis of the fastener bore; wherein the voids are of an approximate trapezoidal cross-section.
 22. Fastening apparatus as claimed in claim 21 wherein said mandrel comprises a radially enlarged head of a maximum diameter which is greater than a diameter of the fastener bore, and less than a maximum external diameter of the fastener shank. 